fix(11): simplify background effects to what VDO.Ninja supports via URL

VDO.Ninja doesn't support custom background images via URL params.
Simplified dropdown to: No Effect, Blur Background, Virtual Background.
Virtual Background (&effects=5) lets VDO.Ninja's own UI handle image
selection. Removed unused SVG/PNG background assets.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mkschulze

companion/index.html

@@ -23,12 +23,7 @@ <h1>JamWide Video</h1>
     <select id="bg-effect" title="Background effect">
       <option value="none">No Effect</option>
       <option value="blur">Blur Background</option>
-      <option value="greenscreen">Green Screen</option>
-      <option value="bg-studio-neon">Studio Neon</option>
-      <option value="bg-concert-stage">Concert Stage</option>
-      <option value="bg-vinyl-wall">Vinyl Wall</option>
-      <option value="bg-synth-wave">Synthwave</option>
-      <option value="bg-jam-room">Jam Room</option>
+      <option value="virtual-bg">Virtual Background</option>
     </select>
     <span id="status-badge" class="badge-disconnected">Disconnected</span>
   </header>